Examining a diamond

ABSTRACT

In order to test whether a diamond has had a layer of synthetic diamond deposited thereon, it is irradiated with high energy ultraviolet radiation to cause emission of luminescence, the luminescence intensity produced by different zones of the diamond being measured and compared. In one embodiment an integrating enclosure (15) is used and the diamond (13) is mounted on a rotatable mount (14).

BACKGROUND TO THE INVENTION

The present invention relates to a method of and apparatus for testingwhether a natural diamond has had a layer of synthetic diamond depositedthereon. This is of particular importance in testing whether the diamondis wholly natural or whether any part of it comprises CVD diamondmaterial and also in locating such material if present.

Synthetic diamond material may be deposited on an uncut or partprocessed natural diamond which is then worked, for example, into around brilliant cut. Alternatively, the synthetic diamond materialcoating may be deposited onto a fully fashioned brilliant stone afterworking of the stone. The thickness of the synthetic diamond materiallayer may be very thin (it could be in the range from 5 microns to 10microns) but the present invention may also be used to detect thickerlayers.

The value of a diamond is in part dependent upon its weight.Accordingly, synthetic diamond material may be deposited onto naturalgem diamonds, before or after cutting of the diamond, to increase theweight of the finished product.

However, the value of a diamond also resides in its qualities ofauthenticity and uniqueness and in the fact that it is an entirelynatural (i.e. mined) product. Thus, a diamond that has not been enlargedby deposition of synthetic diamond material has a value over a diamondwhich has.

Over the years, a number of methods of synthesising diamond materialhave been developed. One of these methods is the chemical vapourdeposition (CVD) technique, which is a low pressure technique involvingdeposition of synthetic diamond (referred to as CVD diamond material inthis specification) onto a substrate from a gas. CVD is the most likelyway in which synthetic diamond will be deposited on a diamond, althoughalternative techniques such as physical vapour deposition have beenproposed. A diamond artificially enlarged by deposition of CVD orsimilar diamond material is referred to in this specification as a"CVD/natural diamond doublet".

CVD diamond material may be deposited on a non-diamond or diamondsubstrate. In the latter case, the CVD diamond material can replicatethe structure of the diamond substrate (referred to as "homoepitaxialgrowth"). The CVD/natural diamond doublet produced can be identical inappearance, density and other common physical properties to an entirelynatural stone and there may be a problem in identifying such aCVD/natural diamond doublet.

It is an object of the present invention to provide a method of andapparatus for testing whether a diamond has had a layer of syntheticdiamond deposited thereon.

It is desired that the apparatus should be simple and may be put intooperation by a person with relatively little training. The method andapparatus should be capable of being operated reliably and consistentlyby a practised jeweler who has no training in laboratory gemologicalanalysis. The method and apparatus should be suitable for screeninglarge numbers of stones, one at a time, and should be suitable forautomation.

British patent application No 9404309.8 discloses a method ofdetermining whether a diamond has had a layer of synthetic diamonddeposited thereon in which the diamond is caused to luminesce withelectrons or high energy ultraviolet radiation and the resulting patternof luminescence is observed to detect zones of superficial syntheticdiamond. Preferably, the whole diamond is irradiated and the patternobserved by eye through magnifying means or on a screen via a CCDcamera.

The present invention provides a method of testing whether a diamond hashad a layer of synthetic diamond deposited thereon, comprising observinga plurality of zones of the surface of the diamond, each zone beingobserved by irradiating the zone with high energy radiation to excite orstimulate emission of luminescence and assessing the intensity of theluminescence.

The present invention further provides an apparatus for testing whethera diamond has a layer of synthetic diamond deposited thereon, comprisinga mounting means, a support for a diamond, movably mounted on themounting means, means for irradiating a diamond supported in the supportwith high energy radiation to excite or stimulate emission ofluminescence, and means for providing a signal dependent upon theintensity of luminescence produced when a diamond mounted on the supportis irradiated. Means for driving the support with respect to themounting means may be provided. The mounting means may be fixed withrespect to the irradiating means.

The invention further provides an apparatus for testing whether adiamond has had a layer of synthetic diamond deposited thereon,comprising an integrating enclosure having a support for a diamond,means for irradiating a zone of a diamond mounted on the support, andmeans for giving a signal dependent upon the flux intensity ofluminescence in the integrating enclosure, produced when a diamond inthe integrating enclosure is irradiated. The support for the diamond maybe movable with respect to the integrating enclosure and may be drivenby drive means.

The inventors have discovered that seeking substantial differences inthe luminescence of different zones of a diamond provides a particularlysimple way to locate superficial layers of synthetic diamond material.No imaging or visual interpretation of a complex image by an operator,as in British patent application number 9404309.8, is required.

By luminescence is meant emitted radiation of a wavelength generallydifferent to the irradiating radiation which causes it.

The luminescence intensity is preferably measured. Preferably, a signaldependent upon the intensity of luminescence from each zone is produced.Alternatively, the surface of the diamond may be scanned by a beam ofirradiating radiation, any significant change in intensity ofluminescence between one zone and the next being detected.

The diamond may be irradiated with ultraviolet radiation of suitablewavelength. Substantially all natural diamonds will luminesce ifirradiated with radiation of wavelength less than 225 nm. It isaccordingly preferable to use radiation of wavelength less than orapproximately equal to 225 nm. The irradiating radiation may besubstantially monochromatic or it may comprise a range or a set ofwavelengths.

It is preferred that preponderantly only the surface region of thediamond is irradiated and caused to luminesce. This is because layers ofsynthetic diamond material may be relatively thin. If the irradiatingradiation penetrates to a depth significantly greater than the thicknessof the thin layer of synthetic diamond material, luminescence could beproduced from underlying natural diamond material which would confuse orswamp out the luminescence from the synthetic diamond layer.

For this reason also, it is preferred that the diamond is irradiatedwith radiation of wavelength less than or approximately equal to 225 nmwhich is very strongly absorbed by all types of diamond. This isdescribed in more detail in British patent application number 9404309.8.

The irradiating radiation may include radiation of wavelengths greaterthan 225 nm. Certain radiation bands of wavelength greater than 225 nmhave different absorption characteristics in different types of diamond.Accordingly, such radiation could penetrate the layer theoreticallybeing studied and cause luminescence in other areas of the diamond,which could confuse the results. Irradiating radiation of wavelengthmuch greater than 225 nm may be confused with luminescing radiation. Itis desirable that radiation of wavelength greater than 225 nm should besufficiently low in intensity that luminescence from parts of thediamond apart from the zone of interest does not swamp out or reduce thecontrast in observations of luminescence. Preferably at least 50% of theirradiation energy is at wavelengths less than 225 nm. Preferably,however, radiation of wavelengths greater than 225 nm should besubstantially excluded by a suitable filter.

The diamond may alternatively be irradiated with a beam of electrons ofsuitable energy, but the apparatus would then be complicated.

The irradiating radiation must be of intensity sufficient to generateobservable luminescence.

The irradiating radiation may be generated by any suitable means, forexample a laser or other source. The irradiating radiation may bedirected onto the gemstone by any suitable means. However, theattenuation of short wavelength ultraviolet radiation by normal opticsis high and it is preferred to use optical equipment which has a hightransmissivity at short ultraviolet wavelengths.

Radiation of wavelengths shorter than 180 nm is attenuated by normal UVoptics and by oxygen in air and is effectively filtered out by theapparatus.

Preferably the radiation is focused onto the diamond. More preferably,the radiation is focused onto an area of the diamond which is smallerthan the total presented surface area of the diamond. Most preferablythe radiation is focused to a small spot and scanned over the surface ofthe diamond.

As set out in more detail in GB 9404309.8, radiation of wavelength lessthan 225 nm is absorbed predominantly in the surface region of thediamond. This is of assistance in the present invention in thatluminescence observed when a given zone is irradiated will bepredominantly dependent upon the composition of the surface of the zoneirradiated.

The luminescence bands observed for various types of diamond (natural orsynthetic) fall within a wide range of wavelengths, generally in thevisible part of the spectrum. A signal dependent upon intensity ofluminescence falling in a relatively narrow band or a relatively wideband may be given. In the latter case, it is preferable to provide acut-off filter to exclude the irradiating radiation.

A synthetic diamond layer deposited upon a natural diamond may beidentifiable if the luminescence thereof is a different colour to theluminescence of the natural part of the diamond or, more importantly, ofa different intensity to the luminescence of the natural part of thediamond. Accordingly, when the plurality of zones are tested,significant differences (for example, the lower signal being of theorder of 80%, preferably 50%, or less of the higher), in the intensityof luminescence produced by different zones of the diamond will suggesta CVD/natural diamond doublet. It is possible that differences inluminescence intensity do not originate in a layer of synthetic diamond.The present invention provides a useful guide. However, further testingmay be beneficial.

It may be sufficient only to test a few zones (maybe only two) in orderto detect a difference in the luminescence in different zones.Preferably, however, a large number of zones are observed.

The intensity of radiation may, in the method of the invention, beassessed by eye. In this case, means should be provided for exluding thehazardous ultraviolet radiation from the observer. If the luminescenceis assessed by eye, it is not necessary to form an image of the zoneirradiated if the irradiating radiation can be confined to the zone ofinterest and irradiation of other zones avoided. In that case, theluminescence, rather than the diamond will be observed in effect.

Preferably, the observed radiation comprises no irradiating radiation. Asmall amount of irradiating radiation may be tolerated in the observedradiation if it does not swamp out luminescence.

The luminesced radiation may be detected by any suitable means. Forexample, a beam splitter may be placed in the path of the irradiatingradiation, being configured to direct luminesced radiation from thediamond to a detector. A filter for filtering out irradiating radiationmay be provided for the detector.

Alternatively, the diamond may be placed in an integrating enclosure anda zone of the diamond irradiated with irradiating radiation. Theintegrating enclosure is provided with a detector for giving a signaldependent upon the intensity of luminescence in the integratingenclosure produced when the given zone is irradiated. The detector mayinclude a filter for filtering out irradiating radiation.

Preferably, the integrating enclosure comprises an integrating sphere.

If an integrating enclosure is used, the zone of the diamond of interestmust be irradiated and substantially no other zones.

The diamond may be irradiated using a beam of confined dimensions whichmay be produced, for example, by an aperture between the diamond and theradiation source.

Preferably, a single zone of the diamond is irradiated at any one timeand a plurality of such zones are irradiated sequentially. However, aplurality of different zones of the diamond may be independentlyirradiated simultaneously and signals dependent upon the intensity ofluminescence produced by each respective zone provided, in succession orsimultaneously, the observations being subsequently compared.

The diamond may be placed with the zone of interest in contact with theaperture, to reduce the inclusion of light from other parts of thediamond. This arrangement is particularly suitable if a beam splitter isprovided in the irradiation path for passing luminescence to a detector.

The confined beam may be of variable dimension or of fixed dimension. Itmay correspond in size to a facet of a worked diamond or to a part of afacet. Preferably, the confined beam is smaller than the maximumdimension of the diamond, or is adjustable in size to allow this. Theaperture may be of size 1-15 mm across, preferably 5-10 mm. An irisaperture may be provided, adjustable in size for best results.

More preferably, the beam may be focussed to a small spot of size 1micron-1 mm across, preferably 5-100 microns, and preferably scannedacross the diamond.

Radiation emanating from the diamond may be passed to the detectorthrough a filter. Preferably, the filter is a cut-off filter forfiltering out the irradiating radiation. A further filter may beprovided for passing selected luminescence bands. For example, a numberof interchangeable filters could be used, each passing light of adifferent wavelength.

The beam is preferably scanned (ie moved continuously or semicontinuously) over the surface of the diamond. Means for scanning thebeam may be provided in the form of means for moving the beam withrespect to the diamond. For example, the diamond may be rotated about anaxis not coincident with the beam of radiation. Preferably, the axis isnormal to the beam of radiation. Means may be provided for moving thediamond linearly with respect to the beam of radiation, for example intwo directions normal to the beam of radiation.

Means may be provided for giving a signal if the intensity of radiationemitted by the diamond changes by an amount exceeding a predeterminedvalue. This is particularly useful if the diamond is scannedcontinuously. It allows changes in surface composition to be readilyidentified. For example, the signal giving means may comprise means forgiving a signal dependent upon intensity of radiation and signalgenerating means for giving a change signal if the intensity ofradiation changes by a given amount. For example, the change signal maybe given if the intensity of radiation measured changes by 5%,preferably by more than 10%, preferably by greater than 20%.

The signal generating means may comprise a timer so that a change signalis only given if the intensity of radiation changes by a predeterminedamount within a predetermined period of time. Means may be provided foraltering the period of time and/or the amount by which the signal mustchange before a signal is given.

In one embodiment of the invention, the diamond is placed in a rotatablemount and rotated continuously, whilst the intensity of luminescence ismeasured. A noisy or modulated DC signal (variations in intensity ofluminescence being caused by naturally occurring slight localdifferences in diamond composition, and internal reflection andrefraction) followed by a much broader pulse of higher or lowerintensity will suggest a CVD/natural diamond doublet.

Preferably, the diamond is rotated a plurality of times in order to givea plurality of readings which may be combined statistically to give astatistically improved reading.

The invention is preferably used with fluorescence--that is,luminescence produced effectively instantaneously by a zone of a diamondwhen it is irradiated with an electron beam or high energy ultravioletradiation.

The apparatus of the invention is preferably confined in a light-tightbox. This is to exclude radiation from external sources from reachingthe detector and to prevent the potentially harmful high energyultraviolet radiation escaping and causing damage to skin and eyes.

The invention will be further described by way of example only withreference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of apparatus for carrying out theinvention, according to a first embodiment;

FIG. 2 is a schematic illustration of apparatus for carrying out theinvention according to a second embodiment; and

FIG. 3 shows a diagram of the signal output obtained.

DETAILED DESCRIPTION OF THE DRAWINGS

In the apparatus generally designated as 1 in FIG. 1, a diamond 2 ismounted in or on a mounting 3 which is rotatable and which istransparent to shortwave ultraviolet light and to visible light. Thediamond is irradiated with ultraviolet radiation of wavelength less than225 nm. The radiation is generated by a source 4 (such as a Xenon flashlamp, deuterium lamp or ultraviolet laser). Irradiating radiation isfiltered through a cut-off filter 5 which removes visible radiation, inorder to improve the contrast of the luminescence observed. Radiation isfocused onto a small zone of the diamond by a lens 6. The small zone ofthe diamond will be caused to luminesce, generating luminescence ofintensity and colour dependent upon the local composition of the zoneirradiated. Some of this luminescence passes back down the direction ofirradiation to beam splitter 7 which passes luminescence through a lenssystem 8 having a filter 9 for removing radiation of wavelength lessthan 225 nm, the luminescence being focused onto a photomultiplier tube10. The photomultiplier tube 10 is connected to a processor 11 andmonitor 12 to display a signal dependent upon the luminescence produced.

In a preferred embodiment of the process of the invention, a pluralityof zones of the diamond are irradiated by fixing the diamond withrespect to rotatable mount 3 and rotating the mount (and the diamond)with respect to the rest of the apparatus so that the point of contactof the radiation moves over the surface of the diamond. The mount isalso movable in a direction normal to the beam and to the axis ofrotation so that the full height of the stone can be scanned by repeatedrotations.

The diamond 2 shown in FIG. 1 is a CVD/diamond doublet, with a layer ofCVD synthetic diamond material on the table of the diamond. The signalfrom the photomultiplier tube 10 displayed on monitor 12 as the diamond2 is rotated will be a "noisy DC" signal as the irradiating radiationpasses over the natural part of the stone, followed by a relativelybroad dip to a lower (or higher) noisy DC signal as the focus of theirradiating radiation moves over the synthetic part of the stone,causing luminescence of a different intensity.

The "noise" will be due to small local variations in diamondcomposition, external and internal reflection and refraction etc.

The apparatus of FIG. 1 is not intended to detect the colour of theluminescence, though it may be modified to do so by providing a numberof exchangeable coloured filters in front of the detector.

The optics used in FIG. 1 are UV transmitting optics such as thosemanufactured by Spindler & Hoyer.

FIG. 2 shows a schematic apparatus for carrying out a method accordingto a second embodiment of the invention. In the apparatus, a diamond 13,which is a CVD/natural diamond doublet, is mounted on a rotatable mountsimilar to the mount 3 shown in FIG. 1. The mount and the diamond areplaced inside an integrating sphere 15 which is lined with a materialwith good reflectance in the visible range. The diamond is irradiatedusing a UV source 16. Light from the source is passed through a filter17 to remove light of wavelengths greater than 225 nm and is focused bya lens 18 onto the surface or near the surface of the diamond 13. Theirradiating radiation is of wavelength less than 225 nm and thereforecauses luminescence. Apparatus for detecting the luminous flux densityof light at the luminescing wavelength(s) is provided in the form of aphotomultiplier tube 19. A filter 20 is provided for filtering out theirradiating radiation and a baffle 21 is provided in the integratingsphere 15 to ensure that the radiation passing to the photomultipliertube 19 is representative of the luminous flux density in the sphere. Aprocessor 22 and monitor 23 is provided for showing the signal producedby the photomultiplier tube 19.

As the diamond 13 is a CVD/natural diamond doublet, the signal producedby the photomultiplier tube 19 when the mount 14 and diamond 13 arerotated is similar to that shown by the monitor in FIG. 1.

FIG. 3 shows in more detail a signal produced by the photomultipliertube 19 or 10 of FIG. 2 or 1 respectively. Fluctuations in the signal("noise") due to natural variations in the diamond are distinct fromchanges in the signal due to layers of synthetic diamond in thatfluctuations are lower in intensity and extend over smaller ranges ofangles of rotation.

In an alternative embodiment, the processor 22 or 11 may be programmedto measure the rate of change of the signal received from thephotomultiplier tube 19 or 10. The processor 11 or 22 may be connectedto means for rotating the mount 3 or 14 respectively. The rate of changeof the signal with respect to time or with respect to position of themount 3 or 14 may be measured. The processing means 11 or 22 may beprogrammed to give a signal if the rate of change of the signal from thephotomultiplier tube 10 or 19 exceeds a given value. A signal is thengiven, for example on monitor 12 or 23 to indicate that a "jump" in theemission of the diamond had been detected. Such a "jump" in emission canbe correlated with the presence of a synthetic diamond layer.

The present invention has been described above purely by way of example,and modifications can be made within the spirit of the invention. Theinvention also consists in any individual features described or implicitherein or shown or implicit in the drawings or any combination of suchfeatures or any generalisation of any such features or combination.

We claim:
 1. A method of testing whether a natural diamond has a layerof synthetic diamond deposited thereon, comprising irradiating a diamondwith a small spot of high energy radiation to excite the emission ofluminescence from the surface of the diamond, scanning in order toseparately sense luminescence emitted by a plurality of different zonesof the surface of the diamond, and determining whether there is asubstantial change in the luminescence from one said zone to another tothereby determine whether there is a layer of synthetic diamonddeposited on the natural diamond.
 2. The method according to claim 1,wherein the diamond is irradiated with radiation preponderantly of awavelength of less than 225 nm.
 3. The method according to claim 1,wherein separate signals are provided dependant upon the luminescenceemitted by the respective different zones.
 4. The method according toclaim 3, wherein the signals are dependant upon the intensity oflumninescence.
 5. The method according to claim 3, wherein differencesin the separate luminescence signals are automatically detected and anindicating signal is generated to automatically classify the diamond ashaving a layer of synthetic diamond deposited thereon if the differencesexceed a predetermined value.
 6. The method according to claim 5,wherein the indicating signal is generated if the intensity ofluminescence from one zone to another chances by a predetermined amountwithin a predetermined period of time.
 7. The method of any of claim 1or 2, wherein whether there is a substantial change in the luminescencefrom one zone to another is determined by eye.
 8. The method accordingto any of the preceding claim 1 or 2, wherein the scanning is effectedby irradiating the diamond with a beam which is smaller than the diamondso that only a zone of the facing surface of the diamond is irradiated,and causing relative motion between the diamond and the beam so that thebeam sequentially irradiate different zones of the diamond.
 9. Themethod according to claim 8, wherein the beam is fixed and the diamondis moved.
 10. The method of claim 8, herein the beam is fixed and thediamond is rotated.
 11. The method of claim 8, wherein the diamond issupported within an intearating enclosure.
 12. The method according toclaim 1 or 2, wherein the diamond is irradiated with a beam providing aspot which is 5 to 100 microns across.
 13. Apparatus for testing whethera natural diamond has a layer of synthetic diamond deposited thereon,comprising:a support for supporting a diamond; means for irradiating adiamond supported by the support with a small high spot of high energyradiation, to excite the emission of luminescence by the diamond;scanning means for providing separate signals dependant upon theluminescence emitted by a plurality of respective different zones of thesurface of the diamond: and means for indicating on the basis of saidsignals whether the diamond has a layer of synthetic diamond depositedthereon.
 14. The apparatus according to claim 13, wherein theirradiating means is for irradiating the diamond with radiationcomprising radiation preponderantly of a wavelength of less than 225 nm.15. The apparatus according to claim 13 or 14, wherein the signals aredependant upon the intensity of luminescence.
 16. The apparatusaccording to any of claim 13 to 14, including means for automaticallydetecting differences in the separate luminescerce signals and forgenerating an indicating signal to automatically classify the diamond ashaving a layer of synthetic diamond deposited thereon if the differencesexceed a predetermined value.
 17. The apparatus according to claim 16,wherein the generating means generates the indicating signal if theintensity of luminescence from one zone to another changes by apredetermined amount Within a predetermined period of time.
 18. Theapparatus according to any of claim 13 or 14, wherein the irradiatingmeans is for irradiating the diamond with a beam which is smaller thanthe diamond so that only a zone of the facing surface of the diamond isirradiated, and the scanning means causes relative motion between thediamond and the beam so that the beam sequentially irradiates differentzones of the diamond.
 19. The apparatus according to claim 18, whereinthe irradiating means are for irradiating the diamond with a fixed beamand means are provided for moving the support.
 20. The apparatusaccording to claim 18, wherein the irradiating means are for irradiatingthe diamond with a fixed beam and means are provided for rotating thesupport.
 21. The apparatus of claim 8, wherein the diamond support iswithin an integrating enclosure.
 22. The apparatus according to any ofclaim 13 or 14, wherein the irradiating means is for irradiating thediamond with a beam providing a spot on the diamond which is 5 to 100microns across.
 23. A method of testing whether a natural diamond has alayer of synthetic diamond deposited thereon, comprising irradiatinggenerally along an irradiation axis a diamond with high energy radiationto excite the emission of luminescence from the surface of the diamond,rotating the diamond about an axis of rotation which is at a substantialangle to said irradiation axis in order to separately sense luminescenceemitted by a plurality of different zones of the surface of the diamond,and determining whether there is a substantial change in theluminescence from one said zone to another to thereby determine whetherthere is a layer of synthetic diamond deposited on the natural diamond.24. The method according to claim 23, wherein separate signals areprovided dependent upon the luminescence emitted by respective saiddifferent zones, differences in the separate signals are automaticallydetected, and an indicating signal is generated to automaticallyclassify the diamond as having a layer of synthetic diamond depositedthereon if the differences exceed a predetermined value.
 25. The methodaccording to claim 23, wherein the diamond is irradiated with a smallspot of high energy radiation to excite the emission of luminescencefrom the surface of the diamond.
 26. A method of testing whether anatural diamond has a layer of synthetic diamond deposited thereon,comprising supporting a diamond within an integrated enclosure,irradiating generally along an irradiation axis the diamond with highenergy radiation to excite the emission of luminescence from the surfaceof the diamond, rotating the diamond about an axis of rotation which isat a substantial angle to said irradiation axis in order to separatelysense luminescence at a luminescence wavelength emitted by a pluralityof different zones of the surface of the diamond, and determining fromany differences in the luminous flux density at the luminescingwavelength within the integrating enclosure whether there is asubstantial change in the luminescence from one said zone to another, tothereby determine whether there is a layer of synthetic diamonddeposited on the natural diamond.
 27. The method according to claim 26,wherein separate signals are provided dependent upon the luminescenceemitted by respective said different zones, differences in the separatesignals are automatically detected, and an indicating signal isgenerated to automatically classify the diamond as having a layer ofsynthetic diamond deposited thereon if the differences exceed apredetermined value.
 28. The method according to claim 26, wherein thediamond is irradiated with a small spot of high energy radiation toexcite the emission of luminescence from the surface of the diamond. 29.Apparatus for testing whether a natural diamond has a layer of syntheticdiamond deposited thereon, comprising a rotatable support for supportinga diamond for rotation about an axis of rotation, a high energyradiation emitter for irradiating generally along an irradiation axis adiamond supported by the support, to excite the emission of luminescenceby the diamond, the irradiation axis making a substantial angle with theaxis of rotation, wherein different zones of the surface of the diamondare irradiated, a detector for sensing the luminescence emitted by saiddifferent zones and providing separate signals dependent upon saidluminescence, and a monitor for displaying said signals, therebyenabling an operator to determine whether the natural diamond has alayer of synthetic diamond deposited thereon.
 30. The apparatusaccording to claim 29, and further comprising a beam splitter on saidirradiation axis, for directing to said detector luminescence emitted bythe surface of the diamond in the direction of the irradiatingradiation.
 31. Apparatus for testing whether a natural diamond has alayer of synthetic diamond deposited thereon, comprising a rotatablesupport for supporting a diamond for rotation about an axis of rotation,a high energy radiation emitter for irradiating generally along anirradiation axis a diamond supported by the support, to excite theemission of luminescence by the diamond, the irradiation axis making asubstantial angle with the axis of rotation, wherein different zones ofthe surface of the diamond are irradiated, a detector for sensing theluminescence emitted by said different zones and providing separatesignals dependent upon said luminescence, and a processor for indicatingon the basis of said signals whether the natural diamond has a layer ofsynthetic diamond deposited thereon.
 32. Apparatus for testing whether anatural diamond has a layer of synthetic diamond deposited thereon,comprising a rotating support for supporting a diamond for rotationabout an axis of rotation, and a source of a fixed beam of high energyradiation for irradiating a diamond supported by the support with a spotof radiation which is substantially smaller than the maximum dimensionof the diamond so that only a zone of the facing surface of the diamondis irradiated, to stimulate emission of luminescence, the fixed beamhaving an axis of irradiation which makes a substantial angle with theaxis of rotation of the diamond wherein when a diamond mounted on thesupport is irradiated and is rotated, the intensity of luminescence ofdifferent said zones can be compared.
 33. The apparatus according toclaim 32, wherein separate signals are provided dependent upon theluminescence emitted by respective said different zones, differences inthe separate signals are automatically detected, and an indicatingsignal is generated to automatically classify the diamond as having alayer of synthetic diamond deposited thereon if the differences exceed apredetermined value.
 34. Apparatus for testing whether a natural diamondhas had a layer of synthetic diamond deposited thereon, comprising anintegrated enclosure, a support for a diamond within the integratingenclosure, the support being rotatable about an axis of rotation, a highenergy radiation source for irradiating successive zones of the surfaceof the diamond along an irradiation axis which makes a substantial anglewith the axis of rotation, while the diamond is rotated, to stimulateemission of luminescence;a filter for filtering out the irradiatingradiation, and a detector behind the filter for providing signalsdependent upon the luminous flux density of luminescence within theintegrating enclosure when the successive zones of the diamond areirradiated, wherein the presence or otherwise of a layer of syntheticdiamond on the natural diamond can be assessed from variations in saidsignals.
 35. The apparatus according to claim 34, wherein separatesignals are provided dependent upon the luminescence emitted byrespective said different zones, differences in the separate signals areautomatically detected, and an indicating signal is generated toautomatically classify the diamond as having a layer of syntheticdiamond deposited thereon if the differences exceed a predeterminedvalue.
 36. The apparatus according to claim 34, wherein the diamond isirradiated with a small spot of high energy radiation to excite theemission of luminescence from the surface of the diamond.